I've never shot Velvia underwater but topside I've shot thousands of rolls and always exposed it at ISO 40.

I think the RVP formula & processing have changed since I was shooting film. I always rated RVP as ISO 40 topside or U/W. U/W rigs: Nikonos V, N90s, or F4 with 2 ikelite Ais (A100). Shooting test rolls is best. Be careful not to over expose transparency film. Many people rate it at ISO 50.

The answer really depends on how the film is processed by whomever is developing the film. I shoot large format velvia on a regular basis and do my own processing. Fuji films require a longer first developing step in the E-6 process than Kodak films. The time recommended for the first developer bath for fuji films is 7:30 vs 6:30 for Kodak films If the developing process used is tailored to kodak requirements, the result is an underexposed transparency by about 1/2-2/3 stop, thus the reason alot of people shoot the film at 40. If the processer you are using knows what they are doing and develops fuji films using fuji requirements, iso 50 works great as the first developer step is longer than that used for kodak. If you shoot at 40 and fuji development timing is used during processing, the result will be overexposed transparencies.

You will be best off shooting some objects around the house and yard and seeing what the results look like prior to doing anything important. You can always ask the people who process your film what recipe they are using as well- 9 out of 10 times, it is the kodak process that is used in my experience- thus my switch to developing my own stuff.

New Fuji velvia 50 is not the same as old 50, so bear that in mind. It was re-engineered in 2007 after it was discontinued in 06 and I've noticed better sat on the old stuff. Exposure results tend be be the same, but there are some nuances to be aware of. Read some other blogs and forums to see what I mean.